The principle of a diffuser and also the use thereof downstream of a turbomachine have been known for decades. Diffusers are utilized in technical terms to convert kinetic energy into pressure energy. For this purpose, the flow must be decelerated. This is generally achieved by means of a continuous or discontinuous increase in size of the flow cross section, which may be realized geometrically in a variety of ways. In fan engineering, diffusers may be used to slow gas flows and increase the gas pressure. Here, in principle, a diffuser constitutes the inverse of a nozzle in that, by contrast to the nozzle, the cross section of the outlet opening is larger than the cross section of the inlet opening, and the cross section of the diffuser interior increases from the cross section of the inlet opening to the cross section of the outlet opening. This applies to gas flows at ultrasonic speeds.
The advantageousness of the use of a diffuser downstream of an axial, diagonal or radial fan is based on the fact that, in general, in all fan construction types, the losses that arise as a result of dissipation of the emerging volume flow are dominant in relation to other loss sources. A part of the flow energy of the emerging jet can, by means of the diffuser, be converted back into static pressure, wherein the increase of the static pressure effects an increase in efficiency. Furthermore, the rotational speed can be reduced for the same air throughput, which entails a reduction in noise.
A diffuser of the type mentioned in the introduction is known for example from EP 0 581 978 A1, which relates to a multi-zone diffuser for an axial-throughflow turbomachine, in which bend angles of the diffuser inlet—both at a hub and also at a cylinder of the turbomachine—are defined, exclusively for the purpose of homogenization of a total pressure profile, by means of the duct height at the outlet of the final blade row. Here, within a deceleration zone of the diffuser, means for eliminating the swirl of the swirling flow are provided in the form of flow ribs, and flow-guiding guide rings divide the diffuser into multiple ducts. In order, with a predefined diffuser area ratio, which is to be understood to mean the ratio of the flow cross sections at the outlet relative to the inlet of the diffuser, and with as small a diameter of the first diffusion zone as possible and with as great a pressure recovery as physically possible and with a swirl-free outflow, to keep the overall length of the diffuser at a minimum, different special embodiments of the first and second diffusion zones are provided for the diffuser, but these require a relatively high outlay in terms of manufacture.
In the case of the embodiment described in EP 0 581 978 A1 as being preferable, the known diffuser is situated in an exhaust-gas housing of a gas turbine, which exhaust-gas housing is designed such that it does not come into contact with the exhaust-gas flow. The actual flow guidance is performed by the diffuser which, in its first zone, is designed as an insert part for the exhaust-gas housing. For this purpose, an outer delimiting wall and an inner delimiting wall of the diffuser are held by means of flow ribs. The outer delimiting wall, which delimits the cross section of the diffuser interior to the outside, forms an outer housing of the diffuser, and the inner delimiting wall, which delimits the cross section of the diffuser interior to the inside, forms an inner housing. The diffuser can thus be considered as being composed of an outer diffuser part, which delimits the flow space to the outside, and an inner diffuser part, which delimits the flow space to the inside.
It is the object of the invention to design a diffuser, an outer diffuser part and/or an inner diffuser part for a diffuser and a fan arrangement of known type such that improvements in the operating behavior of an axial, diagonal or radial fan with regard to efficiency and noise can be attained with little outlay in terms of construction.
Said object is achieved according to the invention in that, along the main flow direction, the cross section of the outer diffuser part changes from a circular cross section at the inlet opening to a non-circular cross section at the outlet opening. Alternatively, in the case of a generic diffuser in which, in addition, an inner diffuser part is arranged concentrically with respect to the outer diffuser part in a known way, and which has a housing which, as an inner housing, delimits the diffuser interior to the inside, the object on which the invention is based is achieved in that the inner diffuser part has, in at least one section perpendicular to the main flow direction, a non-circular cross section about the axis of rotation of the fan.
In a way which is essential to the invention, therefore, there may on the one hand be provided an outer diffuser part for a fan of axial, radial or diagonal type of construction, which outer diffuser part changes, substantially along the main flow direction, from a circular to a non-circular cross section, wherein an inner diffuser part of arbitrary design is provided. On the other hand, there may also be provided in the diffuser an inner diffuser part which, in at least one section, has a non-circular cross section about the axis of rotation of the fan, wherein an outer diffuser part of arbitrary design is provided. The non-circular cross section may be in particular one which is of polygonal, in particular square, basic shape. Here, the invention encompasses the corresponding design of the outer diffuser part and/or of the inner diffuser part.
As in the case of known diffusers positioned downstream of a fan, a diffuser according to the invention effects a pressure conversion from dynamic pressure into static pressure. Here, the speed of the fluid is reduced and homogenized. Whereas a known diffuser designed so as to be fully rotationally symmetrical with respect to the axis of rotation of the fan converts the speed predominantly in an axial direction—that is to say the axial component of the speed vector—into static pressure, and with assumed swirl constancy reduces the circumferential component of the speed only to the extent by which the diameter increases, a diffuser according to the invention additionally converts a part of the circumferential speed of the gas into static pressure, because the non-rotationally symmetrical geometry impedes a movement in a circumferential direction. As a result, the diffuser efficiency advantageously increases.
The diffuser according to the invention, which has a not completely rotationally symmetrical flow cross section as a result of the design of its outer diffuser part and/or inner diffuser part, also offers a further advantage. The maximum installation space available for a diffuser is, in terminal equipment, normally prismatic, and a maximum width, a maximum height and a maximum length are defined by the product. Under these conditions, as a result of the corners present for example in the case of a polygonal, in particular square, cross section, it is possible with a diffuser according to the invention to utilize a larger surface area normal to the axis of rotation of the fan, and thus permit a greater reconversion of the axial speed, by comparison with a completely rotationally symmetrical diffuser. This, too, increases the diffuser efficiency.
Also, in the case of a diffuser according to the invention, owing to the possible utilization of the corners, and without the defined installation space boundaries being crossed, the cross section of the outlet opening has an equivalent radius greater than that of a completely rotationally symmetrical diffuser and can thus achieve an increased pressure reconversion from the circumferential speed. Here, an equivalent radius is to be understood to mean the radius of a circle which has the same surface area as the non-circular diffuser surface. Twice the equivalent radius is also referred to as the hydraulic diameter.
A diffuser according to the invention may advantageously be used together with an axial, radial or diagonal fan operated in particular by means of an electric external-rotor motor, without a follow-up guide wheel having to be provided here.
Here, the overall construction of a fan arrangement with a fan and with a diffuser according to the invention may be of single-part or two-part form. A single-part construction is to be understood here to mean that a static component of the fan, in particular a wall ring, and the entire diffuser—that is to say the inner and/or the outer diffuser part—are formed as a single component. In this way, the wall ring of the fan becomes a constituent part of the diffuser.
A two-part construction is to be understood to mean that the diffuser according to the invention is formed as a component which is separate from the static components of the fan—that is to say as a separate component—wherein said diffuser however can be or is fastened to the static parts of the fan, in particular to the wall ring or to a protective grille, by fastening means such as screws, rivets, clamps etc., or by means of non-positively locking, positively locking and/or cohesive connections, such as by means of a bayonet lock, by means of a clip connection, by means of a welded connection or the like. Here, it is advantageously also possible for a diffuser according to the invention to be designed so as to be suitable for retroactive mounting on an already-installed fan.
Furthermore, in the case of such a two-part construction of an arrangement, it is also possible for the fan itself to be of multi-part construction, which is to be understood to mean that it may also be composed of multiple individual diffuser parts which can be or are connected to one another via fastening means or connections as mentioned above. Here, the diffuser parts which can be assembled in this way at the location of use may be designed in a logistically advantageous manner, such that they can for example be stacked, thus reducing the transport volume. In this way, it is possible for the diffuser geometry, in particular the diffuser length, to be varied according to the requirements through the omission or exchange of individual diffuser parts.
Further advantageous embodiments of the invention will emerge from the following description. The invention will be explained in more detail on the basis of a plurality of exemplary embodiments illustrated in the appended Figures of the drawing: